Those skilled in the art will appreciate the wide utility of electrical field probes in microwave applications. Field probes are typically mounted on waveguides or on resonant cavities to sample a selected portion of the electrical field within these structures to provide a signal to a monitoring circuit or the like. Often, it is desirable to change the phase at which the sampled information arrives at such monitoring equipment. During development, some microwave designers may employ a phase shifter commonly referred to as a "coaxial trombone line". As is commonly known in the art, a coaxial trombone line is basically a "U" shaped structure which is expandable in a manner similar to that of a trombone slide. The sliding action effectively lengthens the propagation path of the sampled electric field energy thereby changing the phase as viewed from the monitoring equipment or other subsequent circuitry that may be employed.
Although satisfactory during the development stage, the coaxial trombone line is too bulky to provide practical use in modern-day microwave transceivers. Accordingly, some microwave designers have replicated the effect of a coaxial trombone line using microstrip technologies. Essentially, two parallel transmission lines are deposited on a substrate. A covering substrate having the "U" connection is placed over the parallel transmission lines in a slideable fashion to control the electrical length along which the sampled field energy must propagate.
However, this approach suffers two severe detriments. Firstly, the microstrip trombone line is difficult to manufacture since mechanical tolerances must be controlled in two dimensions (i.e., both x and y). Should the mechanical tolerance error result in a misalignment of the parallel transmission lines and the slideable plate, the sample field energy may be distorted due to the lossy effects caused by the misalignment. This results in measurement uncertainty and error. Secondly, the microstrip trombone line, though smaller than its coaxial counterpart, still requires a sizeable substrate to implement. Moreover, there must be room allocated to allow the slideable portion to move to control the phase shift. This design is contrary to the modern-day trend towards miniaturization of electronic communications equipment.
Accordingly, there is a need for a miniaturized phase shifter that is simple and inexpensive to build and avoids the detrimental measurement error of the prior art.